[lvm-devel] lvcreate -s - why specifying size for snapshot ?

[Jonathan Brassow]

On Mar 20, 2009, at 11:10 AM, Bryn M. Reeves wrote:

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> Sandeep K Sinha wrote:
>> Hi all,
>> Disk
>> After hunting for relevant documents and failing to find one.
>> I would like to know if its not true that the size of the snapshot
>> should be equal to the original volume?
>> If so, then why do we accept size for a snapshot?
>>
>> Or Am I missing something somewhere else?
>>
>
> The size of the snapshot governs the amount of space set aside for
> storing the changes to the origin volume. E.g. if you made a snapshot
> and then completely overwrote the origin the snapshot would have to be
> at least as big as the origin volume to hold the changes.
>
> You need to dimension a snapshot according to the expected level of
> change. So for example a short-lived snapshot of a read-mostly volume,
> e.g. /usr, would need less space than a long-lived snapshot of a  
> volume
> that sees a greater number of writes such as /home.
>
> Snapshots can be grown after they have been created but you need to  
> make
> sure they don't fill up. If this happens the snapshot will become
> invalid (since it can no longer faithfully track changes on the origin
> volume).

Nice explanation!  This should be placed in the LVM manual.  I will  
notify the appropriate authorities.  :)

  brassow